Neurotropic viruses produce specific patterns of infection in the nervous system by virtue of their ability to invade neurons, replicate in a predictable fashion, and then pass transneuronally through sites of synaptic contact. The goal of this proposal is to exploit the neurotropic properties of alpha herpesviruses for two independent but closely related purposes. The first is to use a well characterized virus, the swine herpesvirus known as pseudorabies virus (PRV), to analyze patterns and mechanisms of viral infection in the mammalian nervous system. The second is to utilize this information to develop viral infectivity as a reliable transneuronal tracing methodology and to apply this method in an analysis of the synaptic organization of functionally distinct components of the visual system. Thr proposal includes two specific aims. The first is to define the neuroinvasive properties of wild type and well characterized mutant strains of PRV introduced into the neuraxis by injection into a variety of central and peripheral sites. The objectives of these studies are to determine the patterns of primary neuronal infection and transynaptic spread produced by inoculation of skin, cranial musculature, vasculature, olfactory mucosa, basal ganglia, and components of the limbic circuitry associated with the hippocampal formation. These areas were chosen because of their functional and anatomical diversity as well as their clinical relevance. In addition, the well characterized gene deletions in these isogenic strains will allow us to identify properties of the virus required for viral invasion and dissemination. The analysis will determine the temporal course of infection produced y each strain, whether infection travels through routes of established connections, if all components of a circuit are equally susceptible to infection, the direction of viral transport through each circuit, and whether susceptibility to infection is influenced by the type of neuron (e.g. sensory vs. motor), its chemical phenotype, or other properties. The second specific aim is to capitalize upon the specific affinity of mutant strains of PRV for functionally distinct components of the visual neuraxis to characterize the synaptic organization of components of the central visual projection systems involved in the photic modulation of hypothalamic function. These strains of virus will be used in combination with classical neuronal tracers and other antigenically- distinct strains of PRV to define multisynaptic circuits devoted to circadian, autonomic and neuroendocrine function. The collective intent of the program is to provide insight into mechanisms underlying viral invasion of clinically relevant neural circuits and to use this information to gain further insight into the functional organization of the nervous system.